The invention relates to a polymeric electrolyte which is part of an electrochemical reference electrode and which is used in potentiometric and amperometric sensors. The polymeric electrolyte is a hydrogel which is stable in a large pH-range and stable against organic solvents.
Electrochemical reference electrodes in combination with potentiometric or amperometric electrodes for the determination of e.g. pH value are known. In most cases so-called combination electrodes are used, wherein the reference electrode is concentrically arranged around the inner sensing electrode. A liquid junction is responsible for the electrolytic contact between the reference electrolyte inside of the reference electrode and the measuring solution. The reference electrode is usually a concentrated aqueous solution of potassium chloride.
There is a transport of material at every liquid junction. If the concentration of materials is not equal on both sides of the liquid junction, diffusion processes occur for creating a balance of all concentrations of material. Additional convective transport of material is present, if there is a pressure difference on both sides of the junction. A third reason for transport of material is migration caused by a potential difference at the liquid junction. The last type of transport only comprises electrically charged particles.
Most of the electrochemical reference electrodes are equipped with a porous ceramic plug serving as a liquid junction. This plug also called diaphragm is mounted in the outer shaft of the electrode either by melting, glueing or pressing. There are different types of diaphragm such as ground glass joints, cotton fibres, twisted metal filaments or wires, porous plastic plugs, wooden pins. In general porous materials can be used for the liquid junction.
A further group of liquid junctions are particularly modified plastics. U.S. Pat. No. 4,002,547 describes a compact consisting of Teflon, glass fibres and KCl powder. During operation some solid KCl is dissolved by the storage or measuring solution resulting in micro pores filled with solution along the glass fibres providing an electrochemical junction to the inner reference electrolyte. These micro pores act as liquid junction. U.S. Pat. No. 5,152,882 describes a further example, wherein glass fibres are embedded in epoxy resin. This composite plastic material is not a liquid junction. After grinding the surface, the glass fibres contact the measuring solution. Due to the lack of a chemical bonding there is a micro gap between the epoxy resin and the glass fibre, which can be filled with solution. Thus, a liquid junction between the inner electrolyte and the measuring solution is formed, as soon as the gaps are filled with solution.
All the above described liquid junctions are characterized by the presence of very fine pores or gaps between the reference electrolyte and the measuring solution. They must be small in order to minimize the exchange of solution between the measuring solution and the reference electrolyte through the liquid junction. If the exchange of solution is too high, the chemical composition of the reference electrolyte changes too rapidly. Parallel thereto, the potential of the reference electrode is subject to change, which causes early recalibration or even sensor failure resulting in a reduced sensor life time.
However, the small pores of the liquid junction may be contaminated rapidly in many solutions. The pores or gaps of the liquid junction may be clogged by substances of the measuring solution or by a reaction between the reference electrolyte and the measuring solution. This contamination changes the surface of the liquid junction and may lead to great adsorption potentials. These error potentials up to 60 mV result in measuring errors up to a 1 pH unit. Especially high errors occur in solutions containing proteins or suspended materials. Generally speaking, chemical reaction or physical adsorption of material at the liquid junction often result in measuring errors. The contamination of the liquid junction or diaphragm represents the most severe problem of potentiometric measurements in laboratory and field.
U.S. Pat. No. 4,959,138 attempts to solve the problem of liquid junction contamination by replacing the many small pores by just one single opening. The liquid or viscous reference electrolyte of conventional electrodes therefore has to be replaced by a polymeric electrolyte to prevent the convective flow of the electrolyte. Hydrogels consisting of crosslinked polyarcrylamide are used as polymeric electrolyte. In order to improve the life time of the reference electrode, the hydrogel additionally contains suspended KCl powder and silicagel. The liquid junction of this reference electrode consists just of a hole filled with polymeric electrolyte. In case of this reference electrode the measuring solution is in direct contact with the polymeric electrolyte. The risk of error potentials due to a contamination of the liquid junction is decisively smaller, since there is only one big opening.
However, the polymer of U.S. Pat. No. 4,959,138 has some severe drawbacks: It shrinks irreversibly in acidic media and the sensor has to be replaced quite often. Furthermore, in the field of pharmaceutical and chemical industry often partially aqueous or non-aqueous solutions occur, i.e. organic solvents are used. The polymeric electrolyte based on polyacrylamide shrinks in the presence of such solvents so that the sensor is no longer functioning. A further disadvantage of the polymer described in U.S. Pat. No. 4,959,138 is its brittleness resulting in cracks after repeated or large temperature changes. These cracks in the polymer might lead to an interrupted electrical circuit. A further severe disadvantage of electrodes with a polymeric electrolyte based on polyacrylamide is the poisoning character of the monomer acrylamide. Working with acrylamide is very dangerous and undesired, particularly, since unreacted monomers remain from each polymerization and may diffuse through the liquid junction into the measuring solution.
U.S. Pat. No. 4,774,029 describes an electrically conductive polymer, which is produced by reacting a tetraalkylammonium acrylate with an acrylic resin and which is impermeable to liquids.
U.S. Pat. No. 5,360,529 describes a reference electrode having a gelled electrolyte solution comprising a polymer containing acrylamide units substituted with a group which lowers the polymer""s tendency to hydrolyse. Preferably used is a polymer based on tris(hydroxymethyl)methyl acrylamide units. Such polymers, however, are unstable in the presence of organic solvents.
It was therefore an object of the present invention to provide a reference electrode, wherein the above mentioned drawbacks of the prior art are eliminated at least to a great extent. On the one hand the reference electrode must withstand contaminations and, on the other hand, however, avoid the problems involved with known polymeric electrolytes. Particularly, the reference electrode shall function also in acidic solutions as well as in the presence of organic solvents and provide correct measuring results
This object is accomplished according to the present invention by providing an electrochemical reference electrode containing an electrolyte comprising a polymer based on monomers selected from N-substituted acrylamides and/or methacrylates, wherein the polymeric electrolytes or hydrogels according to the invention show a considerably higher stability towards acids and organic solvents compared to prior art polymeric electrolytes. Furthermore, N-substituted acrylamides and methacrylates are less hazardous and poisonous substances than acrylamide.
The polymeric electrolyte according to the present invention is obtainable by polymerization or copolymerization of said monomers in a suitable housing of a reference electrode. Preferably the polymerization takes place as usual by adding a suitable radical starter or/and at elevated temperature in the presence of a suitable liquid phase, preferably the liquid phase of the reference electrode. Preferably the polymeric electrolyte is made by dissolving all substances in the reference electrolyte. Apart from the indicated monomers, the polymeric electrolyte may optionally also contain further suitable monomers. Preferably, essentially no acrylamide is applied, i.e. the content of acrylamide is less than 10 Mol %, particularly less than 5 Mol % , more preferably less than 1 Mol % and particularly preferably less than 0.1 Mol % with respect to the total amount of monomers used. Most preferred the polymeric electrolyte is produced without the use of acrylamide and is free of acrylamide. Preferably the content of suitable additional monomers is not more than 10 Mol % with respect to the total amount of the monomers.
The N-substituted acrylamides are preferably selected so that they neither have a fully hydrophilic nor a fully hydrophobic character. Preferred examples are N,N-dimethyl-acrylamide (DMA), N-[tris(hydroxymethyl)methyl] acrylamide N-hydroxymethyl-acrylamide, N-hydroxyethylacrylamide, N-glycerol-acrylamide and combinations thereof. More hydrophilic N-substituted acrylamides such as N-(2-hydroxy-1,1-bis-hydroxymethyl-ethyl)-2-methacrylamide(trisacrylamide) or more hydrophobic N-substituted acrylamides such as N,N-diethyl-acrylamide (DEA) alone are not universally applicable and less suited for certain applications. E.g. trisacrylamide results in an acid resistant hydrogel, however, it has a strong tendency towards shrinking in the presence of organic solvents due to its hydrophobic character. Thus, when used alone, trisacrylamide does not form an electrolyte according to the present invention. DEA is resistant against acids and solvents, the corresponding polymer structure, however, is only little soluble in aqueous solutions. Hence, no homogenous hydrogels are obtained. Such hydrophilic or/and hydrophobic N-substituted acrylamides can only be used in combination with one of the preferred substances, as indicated above. Particularly preferred N,N-dimethylacrylamide is used as N-substituted acrylamide, optionally in combination with one or more further N-substituted acrylamides.
Further, the polymeric electrolyte according to the invention can be based on methacrylate monomers. Preferred methacrylate monomers are methacrylate esters with alcohols having 3 or more hydroxy groups, wherein one hydroxyl group is used for the ester binding with methacrylic acid so that the resulting methacrylate monomer is glycol-methacrylate or 2,3-dihydroxyproplymethacrylate (DPM), respectively. DPM is an unstable monomer, hence it is preferably produced in situ from glycidymethacrylate by hydrolysis. Also preferred are polymeric electrolytes on the basis of a methacrylate ester having at least two free hydroxyl groups, e.g. DPM, and one or more or further methacrylates as comonomers. As comonomers methacrylates with less than 2 free hydroxyl groups may be used.
Further polymeric electrolytes are suitable that contain at least one N-substituted acrylamide and at least one methacrylate as comonomer, wherein in this embodiment also methacrylates containing less than 2 free hydroxyl groups may be used as comonomers. Preferred are e.g. polymeric electrolytes based on N,N-dimethylacrylamide and one or more methacrylates, e.g. 2-hydroxymethylmethacrylate, 2-hydroxyethylmethacrylate (HEMA), 2,3-dihydroxyproylmethacrylate and combinations thereof. The content of methacrylate monomers is preferably 1-30 mol % with respect to the total amount of monomers of the polymeric electrolyte. Further preferred are polymers based on methacrylate ester having at least two free hydroxyl groups, e.g. 2,3-dihydroxypropylmethacrylate and one or more N-subtituted acrylamides as comonomers.
In order to decrease the brittleness suitable substances, e.g. amorphous silic acid or fumed silicon dioxide (Airosil) may be added to the polymeric electrolyte. The amount of fumed silicon dioxide is preferably 0.1-10 wt. % with respect to the total weight of solid particles of the polymeric electrolyte. Fumed silicon dioxide with a specific surface of 50-500 m2/g is preferred.
In addition to hydrogel, the electrolyte of the reference electrode further contains a liquid phase. Preferably the polymeric electrolyte is based on a liquid reference electrolyte. Normally a concentrated aqueous salt solution is used, wherein the salts can be selected from alkali metal salts having anorganic or organic anions. Suitable salts are KCl, NaCl, LiCl, KNO3, KClO4, Na-formiate, Li-acetate and mixtures thereof. An aqueous solution of 3M KCl is preferred.
In certain applications, however, it is advantageous to use a mixture of such a concentrated aqueous salt solution and an organic solvent, which is preferably miscible with the salt solution. Thus, the aqueous salt solution can be replaced by a partly aqueous electrolyte solution containing an organic solvent. Preferred organic solvents are glycerol, ethyleneglycol, methanol, ethanol, n-propanol, isopropanol, acetone and mixtures thereof. Such partly aqueous electrolytes, e.g. 50% (v/v) 3M KCl solution and 50% (v/v) glycerol reduce the risk of a diaphragm contamination by precipitated proteins. In general the amount of organic solvent preferably is in the range from 10-90% (v/v), more preferably from 30-70% (v/v).
The reference electrodes according to the present invention generally can be constructed in any form known from the prior art, e.g. as standard reference electrodes. However, it is preferred to use reference electrodes having an open liquid junction to the surrounding medium. Most preferred the liquid junction consists of one single hole or opening. Thereby problems associated with contamination of the liquid junctions in form of small pores or gaps can be avoided. The direct contact between the polymeric electrolyte and the measuring solution greatly reduces the risk of building up a contaminated liquid junction. The reference electrode according to the present invention is used in combination with a suitable sensing electrode in form of a sensor, e.g. a potentiometric or amperometric sensor in order to determine physico-chemical or electrochemical parameters. This is usally done in a liquid medium such as liquid samples. The reference electrode according to the present invention shows an excellent stability in acidic and in at least partly organic solutions.
The following examples together with two figures are intended to further illustrate the invention.